1. Field of the Invention
The present invention relates to automatic focus adjustment techniques used in image capturing apparatuses and the like.
2. Description of the Related Art
Thus far, focus adjustment apparatuses in video cameras and the like have employed what is known as TV-AF focus adjustment. With the TV-AF method, focus adjustment is carried out by detecting a focus state signal indicating a level of sharpness from an image signal obtained by an image sensor performing photoelectric conversion on a subject image and controlling the position of a focus lens so that the focus state signal reaches a maximum value.
Furthermore, in recent years, techniques that perform focus adjustment by detecting the face of a person from an image obtained through shooting and setting the region corresponding to the detected face as an AF area are being widely used.
For example, Japanese Patent Laid-Open No. 2009-20190 (called “Patent Document 1” hereinafter) discloses an image capturing apparatus that shoots an image after detecting the position of a face in the image and then focusing on the detected face (this will be referred to as “face AF” hereinafter).
Meanwhile, an image capturing apparatus that performs focus detection by setting a focus detection area that includes a region detected as containing a face (see, for example, Japanese Patent Laid-Open No. 2006-227080), an image capturing apparatus that detects the eyes of a person and performs focus detection based on the detected eyes (see, for example, Japanese Patent Laid-Open No. 2001-215403), and so on have also been proposed.
However, with Patent Document 1, in the case where a face has been detected, the face is focused on, whereas in the case where a face has not been detected, the AF area set in a predetermined location of the scene is focused on. An example of a moving picture obtained through this kind of control is illustrated in FIGS. 25A through 25E. FIGS. 25A through 25E illustrate states of focus in the case where a subject enters and exits a scene that is being shot.
In FIG. 25A, no face is detected, and thus the building within a normal frame is in focus. FIG. 25B and FIG. 25C illustrate states in which a subject has entered into the scene and a face has been detected, and the face within a face frame is in focus and tracked while that face is being detected. FIG. 25D and FIG. 25E illustrate images obtained after the subject has left the scene, where the focus gradually returns once again to the building within the normal frame.
When editing a moving picture, it is preferable for editing cut points to occur when there is no movement within the picture. However, even if a moving picture such as that shown in FIGS. 25A through 25E is to be edited by cutting before and after a given image, the timing at which the cut is to be made falls in an image in which the point of focus moves (for example, from FIG. 25A to FIG. 25B, from FIG. 25C to FIG. 25D to FIG. 25E, and so on).
Accordingly, setting a second mode in which focusing is carried out only when a main subject is present in the scene, in addition to a first mode that focuses on the main subject in the case where the main subject is present in the scene and focuses on a subject in the center of the scene in the case where the main subject is not present in the scene, can be considered. However, in the second mode, if multiple subjects are present in the scene, and if the main subject exits the frame and the main subject is then switched to another subject that is then focused on, the focal plane will move and ultimately result in an image that is difficult to cut during editing.
Furthermore, in the focus adjustment disclosed in Patent Document 1, a case in which a moving picture is shot is taken into particular consideration, and as shown in FIG. 23, a sub AF area is set for the entire scene and the focus state signals obtained from the main and sub AF areas are added together at a predetermined ratio and used. This is done in an attempt to maintain a stable state of focus.
However, generally speaking, the faces of people have low contrast, which makes it difficult to obtain a suitable focus state signal, and thus even if the AF areas are set in the manner disclosed in Patent Document 1, there are cases where the face cannot be focused on due to the influence of background contrast components. For example, the example shown in FIG. 23 results in a focus state signal such as that shown in FIG. 24, and thus when a subject B in the background is in focus, the focal plane cannot be moved to a main subject A.